1. Field of the Disclosure
The present invention relates to a holey fiber.
2. Description of the Related Art
A holey fiber (HF) or a photonic crystal fiber is a new type of optical fiber which realizes optical transmission using the principle of total reflection while reducing an average refractive index by having holes arranged regularly in a cladding layer. By using holes for controlling refractive index of optical fiber, the holey fiber can realize unique characteristics which have not been able to be realized by a conventional optical fiber, such as an endlessly single mode characteristic (ESM) and a zero-dispersion wavelength shifted toward a side of an extremely short wavelength. The ESM indicates an absence of cut-off wavelength, and it is a characteristic that enables optical transmission with high transmission rate over a wide band (cf. “Endlessly single-mode holey fibers: the influence of core design,” Optics Express, vol. 13, pp. 10833-10839 (2005) (hereinafter to be referred to as Nonpatent Reference 1), by K. Saitoh, Y. Tsuchida, M. Koshiba, and N. A. Mortensen).
The holey fiber is also expected to be applied to a transmission medium with low optical nonlinearity for use in optical transmission and a fiber laser.
That is, in the optical transmission, for instance, especially in a case of conducting a long-distance terrestrial transmission or an undersea transmission, there is a problem in that a nonlinear optical phenomenon of optical fiber being the transmission medium becomes a major obstacle in conducting the long-distance and high-speed transmission. As a method for solving such problem, use of a large-effective-area (large-Aeff) optical fiber as a transmission medium has been proposed. The large-Aeff optical fiber has a structure in that an effective core-area (Aeff) is enlarged by expanding a core diameter in the optical fiber to be larger than that of the normal optical fiber in order to reduce the optical nonlinearity. For example, “Ultra-low-loss (0.1484 dB/km) pure silica core fiber and extension of transmission distance,” Electronics Letters, vol. 38, pp. 1168-1169 (2002) (hereinafter to be referred to as Nonpatent Reference 2), by K. Nagayama, M. Kakui, M. Matsui, T. Saitoh, and Y. Chigusa, and “Comparisons of merits on wide-band transmission systems between Using extremely improved solid SMFs with Aeff of 160 um2 and loss of 0.175 dB/km and Using large-Aeff holey fibers enabling transmission over 600 nm bandwidth,” Proceeding of OFC 2008, OThR1 (hereinafter to be referred to as Nonpatent Reference 3), by K. Mukasa, K. Imamura, R. Sugizaki, and T. Yagi, discloses an optical fiber of which Aeff is enlarged to 118 μm2 or 160 μm2 in a normal solid optical fiber without having holes formed inside.
However, the solid optical fiber has a problem in that the macrobending loss (bending loss) increases as the Aeff expands. The bending loss is defined as a fundamental propagation mode of light in a case where the optical fiber is bent with a predetermined bend diameter from a state being straight, i.e. the bending loss is defined as an increase amount of transmission loss of LP01 mode.
Such increase of the bending loss can be suppressed by elongating the cut-off wavelength of the optical fiber. However, in order to make an optical fiber operate so that light propagates at a single-mode, there is a restriction in which cut-off wavelength of the optical fiber should be made shorter than a wavelength of the propagating light. Accordingly, the conventional solid optical fiber exhibits a trade-off relationship among realization of low optical nonlinearity (realization of large-Aeff), realization of single-mode operation, and suppression of bending loss.
On the other hand, since the holey fiber can obtain the ESM characteristic, the holey fiber is expected to be able to relax the restriction caused by the trade-off relationship so as to realize the low optical nonlinearly and suppress the bending loss while realizing the single-mode operation.
The solid optical fiber also exhibits a trade-off relationship among realization of large-Aeff, realization of single-mode, and suppression of microbending loss. The microberding loss is defined as an increase amount of transmission loss of LP01 mode due to the optical fiber bending in minute scales due to minute roughness, or the like, on a surface of a bobbin as the optical fiber is wound around the bobbin. In “Reduced microdeformation attenuation in large-mode-area photonic crystal fibers for visible applications,” Optics Letters, vol. 28, pp. 1645-1647 (2003) (hereinafter to be referred to as Nonpatent Reference 4), by M. D. Nielsen, N. A. Mortensen, and J. R. Folkenberg, and “Microbending in photonic crystal fibers—an ultimate loss limit?,” Proceeding of ECOC 2001, We. L. 2. 4. (hereinafter to be referred to as Nonpatent Reference 5), by A. Bjarklev, T. P. Hansen, K. Hougaard, S. B. Libori, E. Knudsen, and J. Broeng, characteristics of the microbending loss of both the holey fiber and the normal solid optical fiber are disclosed. Nonpatent References 4 and 5 relate to theoretical evaluations of the characteristics of the holey fiber and an application of high-power delivery of the holey fiber.
Nonpatent Reference 4 discloses a high power delivery in a visual light range, but does not refer to transmission application. Nonpatent Reference 5 refers to only one type of cross-sectional structure of a holey fiber, and does not specify a cross-sectional structure which is able to sufficiently reduce the microbending loss.